PROJECT SUMMARY Chlamydia trachomatis is the etiologic agent of the most prevalent sexually transmitted infection (STI) in industrialized nations and the blinding condition, trachoma, in under-developed countries. There is an estimated 93 million new reported cases of Chlamydia STI annually worldwide. Trachoma affects approximately 150 million individuals, predominantly women and children. The pathologic hallmark of both diseases is the scarring the results from chronic inflammation; and inflammation at the very early stages of infection is initiated by infection of epithelial cells, and sustained by active bacterial replication and dissemination along the genital and ocular mucosae. Chlamydia is a Gram-negative obligate intracellular pathogen, which means that it requires an intracellular environment for its survival and replication. Hence, invasion of a permissive host cell is paramount to its survival and pathogenesis. In vivo, the primary target is the epithelial cells that line the ocular and genital mucosae. Our overarching hypothesis is that invasion is a Chlamydia-driven process. This pathogen has evolved mechanisms of manipulating the host cell actin cytoskeleton of to induce its uptake, leading to the formation of cell surface structures designed to engulf the bacteria. Invasion involves a number of signaling pathways that in normal cells play a role in regulating actin cytoskeleton dynamics. The bacteria turns on the machinery at its site of adherence, with the location of actin remodeling determined by the restricted translocation of a chlamydial virulence protein called TarP to the cytosolic side of the host cell plasma membrane. TarP recruits a number of signaling molecules to initiate the remodeling of the actin cytoskeleton, followed by the engulfment of the pathogen. Once inside the cell, the pathogen has the opportunity to hijack other cellular processes, including the initiation of inflammation, which when sustained leads to tissue damage and scarring of the ocular conjunctiva the genital tract resulting in infertility. Thus, the ability of Chlamydia to cause disease starts with invasion. While we have a better understanding of TarP function during invasion, mechanistic details on how it is regulated is sparse. This application will investigate the role of mechanotransduction in regulating TarP interactions with host signaling molecules. The following Specific Aims will be addressed. I) To identify the mechanism and significance of TarP mechanosensing in invasion; II) To define the myosin II-regulated components of the chlamydial invasome; and III) To determine the mechanism of uptake post-actin recruitment. The goal is to obtain a detailed understanding of TarP function and regulation in order to guide rational drug and vaccine designs to combat Chlamydia infections.